Turbochargers are required to operate over a wide range of engine speeds and loads. Systems have been developed to precisely control the boost provided by the turbocharger by controlling the exhaust gas provided to the turbine of the turbocharger. In general, a control mechanism for controlling the amount of boost provided by a turbocharger includes some type of variable-geometry mechanism that effectively varies the geometry of the turbine inlet nozzle. Such mechanisms can include, for example, multiple movable aerodynamic vanes in the nozzle, or pistons with or without vanes comprising one wall of the nozzle which are axially movable with respect to a fixed nozzle wall. Control of these mechanisms varies depending on application and can include pneumatic, electromechanical, hydraulic, and electro-hydraulic actuation systems. Control of the actuation system can be open-loop or closed-loop or a combination of open- and closed-loop.
The control of a turbocharger is complicated by the inherent lag in the engine exhaust system and the transient response times of the mechanical elements of the variable-geometry mechanism.
A variable-geometry turbocharger (VGT) such as that disclosed in U.S. Pat. No. 6,269,642 uses vanes to guide the airflow in the turbine nozzle and to adjust the flow area of the nozzle to reduce turbo-lag and improve the acceleration of the engine. The VGT employs an electro-hydraulic actuation system that uses an electrical control signal to activate a spool valve that controls the flow of engine oil into and out of an actuator piston cylinder. The actuator force produced for rotating the vanes is proportional to the pressure differential across the actuator piston cylinder. The dynamic response of the vanes is a function of the oil flow and oil pressure and will vary according to the operating conditions such as supply pressure, hydraulic fluid temperature, ambient temperature and valve loading, among other parameters. These effects are sufficient to slow the dynamic response of the turbocharger vanes. Many different methods are used to attain a faster dynamic response. Internal valve parameters (e.g., nozzle and orifice sizes, spring rate, spool diameter, spool displacements, etc.) may be adjusted to produce a faster response. These changes require additional design, testing, and cost for varying application requirements.
Similar control issues arise with wastegates and other variable-geometry devices in turbocharger applications.
It is therefore desirable to have a control system that improves the dynamic response of the variable-geometry mechanism in a turbocharger.
It is also desirable to have a control system that is applicable to existing variable-geometry mechanisms without modification of the existing components.